lab report covering asymmetric synthesis, Lab Reports of Chemistry

Download lab report covering asymmetric synthesis and more Lab Reports Chemistry in PDF only on Docsity! Abstract: This experiment aims to perform a comparative study of three different methods of asymmetric synthesis: enzymatic chiral resolution, non-enzymatic chiral resolution, and direct chiral reduction of ketones via S-Me-CBS-catalyst on racemic mixtures of 1-phenylethanol and 1-phenylpropanol derived from the achiral reduction of acetophenone and propiophenone. We used HPLC to assess the enantioselectivity of the methods, and we used H NMR to assess % conversion. We found that enzymatic chiral resolution was the most effective method in achieving high enantiopurity (98% to >99.99% ee) for 1-phenylethanol reaction. However, the method was ineffective for the resolution of 1-phenylpropanol (8% to 15% ee), which fits poorly in the enzyme's active site. The non-enzymatic chiral resolution also achieved high enantioselectivity in the resolution of 1-phenylethnaol (87.2% to 94.12% ee). But the method could only achieve relatively low enantioselectivity (14% to 20.8%). However, the difference in enantioselectivity was expected given that the relative kinetics of two alcohols are different. The direct chiral reduction of ketones via S-Me-CBS-catalyst was able to achieve high enantioselectivity in both ketone reductions (88.3% to 87.1% ee). In addition to the three methods of asymmetric synthesis, we also used Mosher’s acid and F NMR to determine % ee of the alcohol products from the ketone reductions. However, Mosher’s acid data (17% to 26% ee) do not reflect the racemic nature of our products, which has a near-zero % ee as confirmed by HPLC. Introduction: Enantiomers are non-superimposable mirror-image structures. They have virtually identical physical properties except for the direction of polarized light. Diastereomers are non- superimposable non-mirror-image structures that possess different physical properties. Enantiomers are important to drug interactions because they behave differently in the chiral environment of the human body. Often, only one of the enantiomers exhibits therapeutic activity; the other can induce undesirable side effects. For instance, thalidomide was a widely used racemic drug that caused congenital disabilities in pregnant women. Ensuing investigation showed that only the R-thalidomide was responsible for the birth defects. If the drug was administered in the form of L-thalidomide, the disaster could have been avoided. Thus, the ability to obtain optically pure drugs is of vital importance. Artificial methods to obtain optically pure compounds can be divided into three categories: chiral resolution, chiral pool, and direct synthesis. Chiral resolution is a well- established practice in industry. The method involves reacting a racemic mixture with chiral derivatizing agents to form a pair of diastereomers separable from each other given their distinct physical properties. Chiral resolution is an example of an enantioselective process where the formation of one enantiomer is favored over the other. In enantioselective processes, chiral auxiliaries, such as chiral derivatizing agents in the case of chiral resolution, are used to bias the formation of one enantiomer. An enantioselective process with near 100% enantioselectivity is considered enantiospecific. Enzymatic chiral resolution is expected to be enantiospecific and non-enzymatic chiral resolution is expected to be enantioselective. In this experiment, we conducted a comparative chiral resolution study involving 1- phenylethanol and 1-phenylpropanol using both enzymatic and non-enzymatic chiral auxiliaries. The racemic alcohols used in our study are produced from the non-enantioselective reduction of acetophenone and propiophenone using NaBH4, an achiral reducing agent. The process is non- enantioselective because the transition state energy of the enantiomers is equal and no pathway is benzylic protons on 1-phenylethanol and 1-phenylacetate, respectively. From the relative integration of the peaks, the % alcohol to acetate was calculated to be 44.1%. The unreacted alcohol is enantiopure as shown by HPLC with a >99% ee. Non-enzymatic chiral resolution The non-enzymatic chiral resolution was successful as supported by the crude IR and 1H NMR. The 3444.95 and 1678.18 cm-1 stretching frequencies correspond to O-H and C=O, respectively, indicating the presence of both 1-phenylethanol and acetophenone. The peaks at 2.63, and 1.54-1.52 ppm correspond to the methyl moiety on acetophenone and 1-phenylethanol, respectively, which further corroborate the presence of both. The calculated % conversion based on the relative integration of the peaks is 54%. The purified alcohol has a % ee of 87.2%. Direct reduction of pro-chiral ketone with chiral (S) - (-) -CBS-oxazaborolidine catalyst The direct asymmetric chiral borane reduction of acetophenone was successful as evidenced by the 1H NMR spectrum which is identical to the racemic alcohol and the IR spectrum showing the presence of the O-H stretching frequency. The product has high enantiopurity with an % ee of 91.0%. (IR) Class data Racemic reduction of ketone with sodium borohydride Overall, the achiral reduction of both acetophenone and propiophenone was successful given that % ees of the products, as expected, are close to zero across the board, with the % ees of 1-phenylethanol ranging from 0.13% to 0.33% and the % ees of 1-phenylpropanol ranging from 0.06% to 0.67%. Esterification with Mosher’s acid The % ees of the alcohols after chiral induction with Mosher’s acid are 17.0% to 26.3 % for 1-phenylethanol and 22.6% to 29.7% for 1-phenylpropanol. The % ees in both cases are significantly different from the expected near zero % ee expected from a racemic mixture. Enzymatic chiral resolution The enzymatic chiral resolution of 1-phenylethanol was confirmed to be enantiospecific (>99% expected % ee), with the % ees of the product ranging from 98% to >99.99%. Furthermore, the enzymatic chiral resolution was effective in converting (R)-1-phenylethanol to 1-phenylacetate as most groups were able to get a >45% conversion of alcohol to acetate (44.1% to 51.95%). The % ees of 1-phenypropanol was significantly lower than that of 1-phenylethanol, with % ees ranging from 9.42% to 15%. Non-enzymatic chiral resolution The non-enzymatic chiral resolution of 1-phenylethanol was, as expected, less enantioselective than enzymatic chiral resolution (~90% expected % ee), with the % ees of the product ranging from 87.2% to 94.12%. The non-enzymatic chiral resolution of 1- phenylpropanol was significantly lower than that of 1-phenylethanol (~15% expected % ee) as expected with % ees ranging from 14% to 20.8%. The reaction conversion of alcohol to ketone was calculated to be 56.0%-70% for 1-phenylethanol and 51.0%-54.1% for 1-phenylpropanol. Direct reduction of pro-chiral ketone with chiral (S) - (-) -CBS-oxazaborolidine catalyst The pro-chiral reduction of both alcohols was expected to be enantiospecific. The experimental % ees ranges from 88.3% to 87.1%. Overall Enzymatic chiral resolution of 1-phenylethanol has the best % ees. Both enzymatic and non-enzymatic chiral resolution are sensitive to the shape of the substrate as evident by the difference in the % ees of 1-phenylethanol and 1-phenylpropanol products. Both Mosher’s acid chiral induction and direct reduction of pro-chiral ketone are not sensitive to the shape of the reactant as there is no significant difference in the % ee of two alcohols. Discussion/Conclusion: In this experiment, we conducted a comparative study of different methods of asymmetric synthesis. We found that enzymatic chiral resolution was the most effective method to achieve enantiopurity. Mosher’s acid data do not show the mixtures to be racemic. We believe that is caused by incomplete esterification, which is confirmed by the residual O-H stretching frequencies in the IR spectrum. As expected, enzymatic chiral resolution was extremely sensitive to the shape of the substrate. This was reflected in % ees of the two alcohols. The % ees of 1-phenylethanol ranging from 98% to >99.99% indicate that the enzyme is enantiospecific. However, in the case of 1- phenylpropanol, the addition of a methyl moiety led to a drastic drop in the % ees of the alcohol (9.42% to 15%). It is worth noting that there is no significant difference in the % conversion from alcohol to acetate for both alcohols, indicating that the additional methyl moiety did not affect reaction kinetic. We therefore conclude that the additional methyl moiety hindered the enzyme’s ability to recognize the correct chiral substrate resulting in poor chiral resolution. This further confirms that the enantioselectivity of the enzyme is highly dependent on the fit of the substrate in the active site. The non-enzymatic chiral resolution is enantioselective. As predicted by the relative rate of oxidation of each enantiomer, the % ees of 1-phenylethanol are about 90% and those of 1- phenylpropanol hover around 15%. We expected the % ee of 1-phenylpropanol to be significantly lower than that of 1-phenylethanol because the relative rate of oxidation to ketone (crystal, a small piece) added to the reaction. The reaction was sealed with parafilm and allowed to react for a week at r.t. After completion, the reaction was quenched by adding water (2 mL), after which the mixture was transferred to a separatory funnel and 1M HCL (5 mL) was added and mixed in thoroughly. The aqueous was extracted with DCM (5 mL). The combined organic layer was then extracted with sodium bicarbonate, dried over anhydrous sodium sulfate, and concentrated down to yield the product as a colorless liquid. TLC (50% EtOAc/hexanes): alcohol (0.2), ketone (0.6) IR (cm-1): 3339.41 (O-H), 2968.08 (C-H), 1721.17 (C=O) 19F NMR (400 Hz) in CDCl3: (ppm), -71.4 (s, 1F), -71.6 (s, 0.71 F) (diastereomers) % ee Mosher ester: 17% ee Enzymatic chiral resolution To a 20 mL reaction vial was added 1-phenylethanol (50 mg, 0.4 mmol), hexanes (8 mL), and vinyl acetate (150 µL) by pipetman in this order. The mixture was stirred until all was in solution, after which lipase B (80 mg) was added. The reaction was placed in a heating block at 35 °C stirred for 1.5 hr. The reaction was filtered through a fritted column to remove the lipase B. TLC (20% EtOAc/hexanes): acid (0 rf), alcohol (0.3 rf), ester (0.5 rf) 1H NMR (400 Hz) in CD2Cl2: (ppm), 5.88-5.88 (q, 1H) (acetate), 4.91-4.87 (q, 0.79H) (alcohol) % alcohol to acetate: 44.1% % ee chiral HPLC: >99.99% Non-enzymatic chiral resolution To a 50 mL Erlenmeyer flask was added 1-phenylethanol (611 mg, 5.0 mmol), DCM (10 mL), and aq. KBr solution (15 mL). The biphasic mixture was stirred, followed by the addition of catalyst I (65 mg, 0.10 mmol). The mixture was stirred further for 5 mins, after which PhI(OAc)2 (0.966 g, 3.0 mmol) was added and the reaction was allowed to proceed at r.t. for 15 mins. The reaction was quenched with saturated sodium thiosulfate (20 mL). The mixture was then transferred to a separatory funnel, and the flask was rinsed with DCM (10 mL) to ensure maximal transfer. Afterward, the aqueous layer was extracted with DCM (20 mL) two times. The combined organic layer was extracted with equal volume brine, dried over anhydrous sodium sulfate, and concentrated down to yield the crude mixture was a colorless liquid. The crude was purified via silica chromatography. The alcohol product was eluted in 30% EtOAc/hexanes. The concentrated product was a colorless liquid. TLC (20% EtOAc/hexanes): alcohol (0.3 rf), ketone (0.5 rf) IR (cm-1): 3444.95 (O-H), 2975.90 (C-H), 1678.18 (C=O) 1H NMR (400 Hz) in CD2Cl2: (ppm), 2.63 (s, 1H) (ketone), 1.54-1.52 (d, 0.85H) (alcohol) % reaction conversion to ketone: 54% Purified product 1H NMR: identical to racemic alcohol % ee chiral HPLC: 87.2% Direct reduction of pro-chiral ketone with chiral (S) - (-) -CBS-oxazaborolidine catalyst The reaction was conducted under nitrogen flow. To a 20 mL reaction vial was added chiral catalyst (0.10 mL, 0.05 mmol) and borane-N,N-diethyaniline complex (1.2 mL, 1.2 mmol). Afterward, ketone solution (2.0 mL, 1.0 mmol) was added dropwise. The reaction was stirred at r.t. for 1 hr. The reaction was quenched by dropwise addition of methanol (1 mL). 1M HCl (5mL) was added to the quenched reaction and allowed to stir for 15 min at r.t. The reaction mixture was transferred to a separatory funnel. The aqueous layer was extracted with diethyl ether (10 mL). The combined organic layer was extracted 1M HCl (20 mL), followed by water (20 mL) and brine (20 ml), after which, it was dried over anhydrous sodium sulfated and concentrated down. TLC: (20% EtOAc/hexanes): alcohol (0.3 rf), ketone (0.5 rf) 1H NMR: identical to racemic alcohol % ee chiral HPLC: 91.0% References: (1) J. Chem. Educ. 1996, 73, 481 (2) Course Manual (3) J. Chem. Educ. 2017, 94, 800–805 Figure 1: enzymatic chiral resolution